(i) Field of the Invention
This invention relates to a method of producing optical fiber by a chemical vapor reaction. More particularly, it relates to a method of producing optical fiber of excellent transmission characteristics by suppressing degradation in the refractive index of the central part of the optical fiber, the degradation being one of the problems inherent to the method of producing optical fiber exploiting a chemical vapor reaction.
(ii) Brief Description of the Prior Art
Among methods of producing optical fiber which is a transmission medium for the optical communication, the so-called CVD techniques wherein a chemical vapor reaction is exploited possess such a large number of merits that optical fiber having any desired distribution of refractive indices can be fabricated and having low transmission loss can be obtained.
As one of the CVD techniques, the MCVD technique is known wherein a cladding and a core or a glass film to serve as a core are deposited onto the inner wall surface of a glass tube made of quartz or the like, the glass tube is subsequently heated to collapse its hollow portion, thereby to make an optical fiber preform in the shape of a rod, and the optical fiber preform is heated and drawn into the optical fiber. One of problems in this MCVD technique is that, since the deposited glass film contains, besides for example SiO.sub.2, a dopant for controlling the refractive index such as B.sub.2 O.sub.3, TiO.sub.2, P.sub.2 O.sub.5 and GeO.sub.2, the dopant added to the glass film vaporizes in heating and collapsing the hollow glass tube with the deposited glass film into the solid rod. The vaporization of the dopant results in lowering in the refractive index at the central part of the optical fiber and exerts bad effects on the transmission characteristics of the optical fiber.
In a prior-art method of manufacturing optical fiber on the basis of the MCVD technique, a core or a cladding and a glass film for a core is/are deposited to a predetermined thickness on the inner wall surface of a glass tube by exploiting a chemical vapor reaction. Subsequently, the glass tube having the CVD glass film is heated and collapsed, so that the hollow portion may become solid, into a preform in the shape of a rod. The preform thus obtained is heated and drawn into the optical fiber having an outside diameter of about 100 microns. In general, the refractive index distribution in a section of the optical fiber is similar to that in a section of the preform. The thermal diffusion etc. of the dopant hardly takes place in the drawing step of manufacture. The degradation of the refractive index at the central part of the optical fiber fabricated by the CVD process has occurred already in the preform making step. The lowering of the refractive index at the central part of the optical fiber will be ascribable to the fact that, as previously stated, the dopant vaporizes from the surface of the glass for the core in the step of heating and collapsing the hollow glass tube after the chemical vapor deposition. According to a study by the inventors, the dopant is susceptible to vaporization at the heating temperature during the collapsing and the density of the deposited glass film. This is considered to verify that the dopant vaporizes from the inner wall of the hollow glass tube subjected to the CVD, due to the heating during the collapsing, and that it diffuses within the CVD glass film.
In a prior-art method of collapsing, the hollow glass tube is collapsed into the solid rod in such a way that the feed of a reaction gas into the glass tube is stopped upon completion of the CVD, and that while letting an oxidizing or inert gas to flow into the tube or after filling the interior of the tube with the gas, the heating temperature is further raised. In this case, the glass film subjected to the CVD at a comparatively low temperature has not undergone a sufficient densification. When, in that state, the glass film is immediately heated by the raised temperature of the collapsing step, the dopant vaporizes in large quantities and over an extensive area of the CVD film. As the result, the optical fiber preform based on the prior-art method involves an extensive and considerable lowering of the refractive index at the central part thereof and has bad influences on the transmission characteristics of the optical fiber drawn therefrom.
As known techniques close to this invention, there are inventions disclosed in the official gazettes of Japanese Patent Application Public-disclosures Nos. 50-120352, 50-51338 and 51-3650.